This invention relates generally to electromagnetic flowmeters, and more particularly to a flowmeter whose magnet system includes at least one solid ferromagnetic core welded to the outer surface of the metal flow pipe.
An electromagnetic flowmeter is adapted to measure the volumetric flow rate of a fluid, particularly fluids which present difficult handling problems such as corrosive acids, slurries, detergents and the like. In a meter of this type, a magnetic field is generated which is mutually perpendicular to the longitudinal axis of the flow tube through which flows the fluid to be metered and to the axis of the meter electrodes.
Since the velocity of the fluid is directed along the longitudinal axis of the flow tube, the voltage induced within the fluid will be perpendicular to both the velocity of this fluid and the flux linkages of the magnetic field. Thus the metered fluid consitutes a series of fluid conductors moving through the magnetic field. The more rapid the rate of flow, the greater the instantaneous value of voltage developed at the electrodes, thereby producing an output signal representative of flow rate.
As noted in the Mannherz et al U.S. Pat. No. 3,596,104, one well-known type of electromagnetic flowmeter in widespread commercial use includes a stainless steel flow pipe whose inner surface is lined with an insulating material such as Teflon, neoprene or vitreous enamel, depending on the anticipated nature of the fluid being measured. Two electrodes are mounted at diametrically-opposed positions on the central portion of the flow pipe, the electrodes being insulated from the metal pipe by bushings. The faces of the electrodes are flush with the inner surface of the insulating liner and come in contact with the fluid being metered.
As noted in the Seebode, et al U.S. Pat. No. 3,926,049, to overcome polarizing effects, it is known to excite the magnet system of an electromagnetic flowmeter with an alternating current. However, as a consequence of such AC excitation, undesirable eddy currents are generated.
In the conventional electromagnetic flowmeter of the type described for example in the Mannherz et al patent, the magnet system to establish the magnetic field consists of two saddle-shaped magnet coils surrounded by a box-shaped laminated iron core which in turn completely encompasses the central portion of the flowmeter, the laminated core serving to minimize eddy current effects.
In electromagnetic flowmeters whose magnet system makes use of laminated cores, the core laminae are glued or otherwise held together, and the magnet system is screwed onto the metal flow tube by means of holders. In the course of operation, the laminae of the core tend to be set into mechanical oscillation at the frequency of the excitation current. As a result, there is danger that the laminae of the core may be loosened and displaced relative to each other and with respect to the flow tube to which they are attached by holders, thereby producing objectionable noise. Moreover, temperature effects arising from the temperature of the fluid being measured may also serve to displace the magnet system.
As a consequence of such displacement the calibration of the magnet systems may be impaired and cause spurious interference signals to be generated which degrade the accuracy of the meter. In order to minimize these adverse effects, special expedients have heretofore been provided for mounting the magnet system on the flow tube to obviate displacement thereof. Such expedients not only complicate the structure of the meter, but are quite expensive.